This invention relates to conductive paste, etc. to be used for mounting to substrates of electronic parts or inter-electrode joint, etc.
Conventionally, conductive adhesive agents which have been used for mounting onto substrates of electronic parts or for inter-electrode joint, whose purpose is fixation and electrical connection, therefore, generally have comprised metal particles such as conductive silver and copper, etc., having been made to disperse in a viscous resin for fixation or cure-type resin paste.
To make joint effective, it is important that an adhesive agent and the surface of adherend gets sufficiently closer on a micro level. In case of an adhesive type, resins which are flexible, easily deformed and has high affinity with the surface of adherend have been used, while in case of a cure type, resins which have high fluidity and wetness has been used. This enables an adhesive agent to enter the recess and salient of adherend having unit of a micro level and attain good joint in case of either adhesive type or cure type.
On the other hand, for conductive particles, particles in various shapes have been used according to their use, those of adhesive type as well as cure type have been required to be with low resistance so that good electric connection is obtainable, and have been arranged so as to improve the content ratio of conductive particles in the adhesive agent, and remove bubbles, and increase volumetric occupation ratio of conductive particles.
In addition, as concerns cure type, because volume shrinkage takes place at the time of curing, for the purpose of relieving the resultant internal stress, addition of plasticizers, etc., has been implemented as well.
In general, an electrical joining method involving conductive adhesive agents is being used in various fields for its simple nature, and recently, since resins themselves or additions of plasticizers show flexibility, the method is being in wide use as an electric connection method for portions where thermal as well as mechanical strain is apt to occur. In addition, the electric joint method using conductive adhesive agents is remarkably applied and extended to terminal electrodes for small-sized electronic parts and mounting of electronic parts onto substrates. As an example, an embodiment of mounting onto substrates has been disclosed in Japanese Patent Laid-Open No. 1-232735 specification.
However, in case of conventional conductive adhesive agents, whose conductivity is maintained by contact among conductive particles, there was a problem that stress was generated when expansion and shrinkage took place due to heating or when the substrates were bent, and at the time of stress relief thereof the contact among particles was removed to heighten resistance.
In addition, since electric connection to electrodes to be connected has been conducted with contact between conductive particles and electrodes, there has been a problem that not only being weak against stress as described above but also in some cases, surface resistance between electrodes and adhesive agents would become greater from the initial period.
On the other hand, in recent years, in correspondence with miniaturization as well as higher performance of electronic equipment, not only for industrial use but also in a vast field of equipment for commercial use, it has been strongly desired that multilayer wiring circuit substrates on which semiconductor chips such as LSI, etc., can be mounted with higher density are supplied. In such a multilayer wiring circuit substrate, it is important that electric connection can be attained with highly connection reliability among wiring patterns of plural layers formed with fine wiring pitch.
To comply with such demand from the market, in stead of metal-plated conductor on the inner wall of through hole, which has been the main stream of inter-layer connection of conventional multilayer wiring substrate, there is an inner via hole connection method that enables optional electrode on a multilayer print wiring substrate to be interlayer-connected in optional wiring pattern position, that is, whole layer IVH structural resin multilayer substrate (Japanese Patent Laid-Open No. 06-268345 specification). This one, in which conductive body filling in via holes of a multilayer print wiring substrate can connect only respective necessary layers and inner via hole can be disposed right under the part land, thus, can realize miniaturization of substrate sizes as well as mounting with high density.
However, a general inner via hole connection method is implemented by squeegee-printing the conductive paste which fills in inside the via holes, and especially for filling conductive paste toward via holes with micro diameters the viscosity characteristics of conductive pastes are supposed to play an important role. In conductive pastes for via hole filling, printing characteristics and conductivity in inter-layer connection oppose each other from the point of view of structural materials.
That is, when composition ratio of conductive particle among fine particles is increased for the purpose of improving conductivity, resin components are absorbed on the surface of conductive particle, resulting in increase in viscosity of the paste, and thus printing becomes less easy. In addition, when conductive particles with small surface area, that is, greater particle diameter are used, or structural ratio of conductive particles is decreased for the purpose of lowering viscosity of the paste to improve printing characteristics, contact surface area between conductive particles themselves becomes smaller, thus a problem on connection reliability occurs. That is, as concerns conductivity in a conductive paste, with conductive particles coming into point contact each other, and since electric connection between electrode foils disposed on the upper surfaces of via holes and conductive particles depends on point contact between conductive particles and the electrode foils as well, thus, when in particular the wiring pattern in terms of line width as well as distance between lines have been made minute, accordingly diameters of via holes have been made minute as well, it becomes extremely difficult to secure high connecting reliability in inter-layer connection among multilayer wirings.
The purpose of the present invention is to solve issues such as the foregoing to provide conductive paste, etc., which has low resistance and can tolerate stress so as to enable the conductive structure to be formed.
First and foremost, conductivity of, for example, an adhesive agent, which is a typical example of conductive paste, is given by mutual contact of conductive particles dispersed in the adhesive agent. In addition, electrical connection with electrodes to be connected is given by contact between conductive particles in the adhesive agent and electrodes. Accordingly, as the contact area between conductive particles as well as the contact area between conductive particles and electrodes is greater, better electric connection with smaller resistance can be obtained.
In a conductive adhesive agent, foamable resin powder, for example, is contained as a foamable material, and when the foamable resin powder foams by heating or decompression, as a macro structure, the volume content ratio of conductive particles in the adhesive agent decreases. However, micro-structurally, contact area between conductive particles as well as between conductive particles and electrodes can be made equal to or more than the contact area in the case where no foamable resin powder is contained. This can be attained by adopting a method of construction such as in the method of the present invention, that the foamable resin powder undergoes foaming prior to curing of the resin, and subsequently undergoes curing in the case where the cure type of the conductive adhesive agent of the present invention is used in such a place that it can expand freely. At that time, the contact between conductive particles, which is defined by hardening shrinkage of resin in the portion other than the air bubble created by foaming, changes little compared with in the case where foamable resin powder is not contained, and in addition the number of joint points between conductive particles is not significantly decreased. Therefore, the contact area between conductive particles for the entire layer of the conductive adhesive agent will become approximately same. This will increase in the entire volume of the layer of the conductive adhesive agent to be increased in apparent specific resistance, but the value of the resistance will not increase much, giving rise to those being conductive approximately on the same level.
On the other hand, in such the case where a conductive adhesive agent is inserted in the gap having a fixed distance to undergo curing, when a conventional conductive adhesive agent is used, the spacing regulation will hamper the hardening shrinkage for the entire conductive adhesive agent layer, and hamper the normally expected increase in the contact area between conductive particles by the hardening shrinkage. Such a case will only end with a simple expansion or give rise to gaps, resulting in the connection with high resistance values. In contrast,when a conductive adhesive agent of the present invention is used, the bubble portions expand more than the conductive portions other than bubbles hardening shrinkage, and thus, compared with in a conventional case, in a limited space, stronger compression stress is generated and raises pressure in the portions other than bubbles. This increases contact area between conductive particles and between a conductive particle and an electrode to which the particle is attached more than in the conventional ones without generating any gaps to make it possible to attain lower resistance with a good joint condition. In this case, the apparent specific resistance also becomes smaller.
This serves to make it possible to provide adhesive agents having conductivity equal to or more than those in the case where no foamings take place as far as content ratio of foamable resin powder to be included does not become too excessive.
At the same time thereof, in the case where the adhesive agent has been made to contain foamable resin powder, the structural body after the foaming will become a network structure having a lot of small pores internally and comprising conductive particles and adhesive agent resin. A structural body having a number of pores is extremely abundant in flexibility and can be easily deformed against stress to relieve stress. This stress relief is proceeded mainly by deformation of pores, the strength to set contact between conductive particles as well as joint with electrodes apart greatly damps. Accordingly, also when strain has been given, it becomes possible to continue holding good electrical connection.
As described above, with foamable material being contained in conductive adhesive agent, it becomes possible to provide conductive paste which shows good strain-stress resistant characteristics and has low resistance.
In order to attain the above-described objects, provided is the conductive paste of the present invention which contains conductive particles, foamable materials foaming at the time of heating or depressing, and resin, and maintains good conductivity as an electrical connection material even after the expansive materials expand, that is, the conductive paste of the present invention is characterized by being able to maintain the resistance ratio per volume which is not less than 10xe2x88x926 xcexa9*cm and not more than 10xe2x88x922 xcexa9*cm.
In addition, the present invention covers conductive structures using such conductive paste, electronic components, mounting bodies, circuit substrates, an electric connection method, forming method of circuit substrate, and forming method of ceramic electronic components.